1. Field of the Invention
The present invention relates to a light beam scanning device and an image forming apparatus, and more particularly to light amount control performed by an image forming apparatus, such as a laser beam printer, in scanning a photosensitive member using a plurality of light beams.
2. Description of the Related Art
In recent image forming apparatuses provided with a light beam scanning device, with a view to increasing the printing speed and enhancing the resolution of output images, the number of light emitting elements (light emitting points) provided in a light source, such as a semiconductor laser, is increased, whereby an image is formed by simultaneously scanning a photosensitive member using a plurality of light beams.
The image forming apparatuses that scan a photosensitive member using a plurality of light beams are capable of achieving higher printing speed and higher resolution of output images without increasing the rotational speed of a polygon mirror, compared with image forming apparatuses provided with a single light emitting element and increased in the rotational speed of a polygon mirror.
In increasing the number of light beams, it is envisaged to use a vertical cavity surface emitting laser (hereafter referred to as the VCSEL). The VCSEL is easier to increase the number of laser beams than an edge emitting laser (hereinafter referred to as the EEL).
The VCSEL emits light only in a direction perpendicular to a chip surface, and hence a VCSEL element cannot incorporate a photodiode for detecting a light amount of a light beam. For this reason, each image forming apparatus that uses the VCSEL as a writing light source is required to have a photodiode sensor provided outside the VCSEL. Based on the light amount of a light beam received and detected by the photodiode sensor, automatic power control (APC) of the light beam emitted from the VCSEL is performed.
As a method of performing the automatic power control based on the received light amount detected by the photodiode sensor, it is known to dispose one photodiode on a moving line (scanning line) along which a plurality of light beams converted to scanning light by the polygon mirror are scanned, and control the light amount of each of the light beams based on the amount of light received by the photodiode (Japanese Patent Laid-Open Publication No. H09-230259). This publication discloses a beam writing device which uses a plurality of light beams of which the width of a space interval in a moving direction (scanning direction) therebetween is made wider than the width of the photodiode so as to prevent more than one light beam from simultaneously entering the one photodiode. According to this publication, a result of light reception by the photodiode corresponds to an amount of one light beam since more than one light beam does not simultaneously enter the photodiode, which makes it possible to perform light amount control of each light beam with accuracy.
However, when the image forming apparatus is configured as in Japanese Patent Laid-Open Patent Publication No. H09-230259, it is required to make wider the width of space interval between adjacent light beams in the scanning direction thereof than the width of the photodiode. Then, it is required to increase the distance between light emitting elements that emit the adjacent light beams, which increases the size of the light source of the image forming apparatus.
On the other hand, if a plurality of light emitting elements are disposed such that the space interval between each adjacent ones of light beams in the scanning direction thereof is made narrower than the width of a photodiode, and the automatic power control is performed by lighting the plurality of light emitting elements in the order of arrangement thereof, adjacent light beams simultaneously enter the photodiode, which makes it impossible to perform light amount control of each light beam with accuracy. Let us consider, for example, a case where a light beam from a first light emitting element and a light beam from a second light emitting element are adjacent to each other in the scanning direction of light beams, and the light beam from the first light emitting element scans a photosensitive member, immediately before the light beam from the second light emitting element. When performing the automatic power control of the second light emitting element immediately after executing the automatic power control by causing the first light emitting element to emit the light beam, the automatic power control of the second light emitting element cannot be executed before turning off the first light emitting element. If the automatic power control of the second light emitting element is started before the first light emitting element is turned off, the light beam from the first light emitting element and the light beam from the second light emitting element enter the photodiode. A result of light reception detected by the photodiode contains a light mount of the light beam from the first light emitting element, and hence if the automatic power control is performed based on the result of light reception by the photodiode, the light amount of the light beam from the second light emitting element cannot be controlled to a predetermined light amount.
It can be envisaged that the automatic power control of the second light emitting element is started after waiting for the first light emitting element being subjected to the automatic power control to be completely extinguished. However, at the time of starting the automatic power control of the light beam from the second light emitting element, the light beam has already come to a point where it has entered the photodiode, and there is not sufficient time to complete the automatic power control before this light beam terminates scanning of the photodiode.